1. Field of the Invention
The present invention relates in general to a mass flow controller for controlling gas flowrate, and more particularly to a sensor device of such a mass flow controller suitable for stably sensing gas flowrate of the mass flow controller regardless of variation of peripheral temperature.
2. Description of the Prior Art
With reference to FIG. 1, a known mass flow controller (Hereinafter, referred to simply as "MFC"), a kind of device for controlling gas flowrate of a manufacturing system, such as a semiconductor manufacturing system, generally comprises a main body 1 which is provided with inlet fittings 2 and outlet fittings 3 at its opposite ends. To a position of the main body 1 near the inlet fittings 2, a sensor device 4 is mounted for sensing inlet gas flowrate, whilst to a position near the outlet fittings 3, a gas flowrate control valve 5 is mounted. The MFC further includes a circuit board 6 for supplying the sensor device 4 with electric power supplied through a connector 7, or an output terminal.
Turning to FIG. 2, there is shown a detailed construction of the known sensor device 4. As depicted in this drawing, the sensor device 4 has a case 12 covered with a heat insulation plate 11. A sensor tube 13 is provided inside the case 12 such that its inlet and outlet 13a and 13b are protruded from a side of the case 12, respectively. In order to fix the position of the sensor tube 13 in the case 12, the inlet 13a and the outlet 13b of the tube 13 are brought into engagement with individual O-rings 16 and 16' at the outer surface of the case 12. The sensor device 4 further includes an up-stream coil 14 and a down-stream coil 15 which coil around the sensor tube 13 and sense quantity of heat of the gas passing through the tube 13, respectively.
In the gas flowrate sensing operation of the aforementioned sensor device 4 of the MFC, about 5 cc ( this amount may be changed in accordance with styles of the MFC) of inlet gas, which is received by the inlet fitting 2 of the main body 1, is introduced to the inlet 13a of the tube 13 and flows through the tube 13 in a direction from the up-stream coil 14 to the down-stream coil 15.
At this time, the gas in the sensor tube 13 flows at a velocity which is generally proportional to the gas flowrate in the main body 1 of the MFC. Otherwise stated, the current velocity of the gas 4 in the sensor tube 13 is increased when the gas flowrate in the main body 1 is increased, on the contrary, the velocity is reduced when the gas flowrate in the main body 1 is decreased. In addition, it has been noted that all kinds of gases have individual specific heats which are different from each other in accordance with their flowrates and kinds. The heat of a gas is proportional to the current velocity of that gas.
In accordance with the aforementioned theory in the gas flowing, it is well known to those skilled in the art that the temperature difference (.delta.T=T.sub.1 -T.sub.2) between the heat T.sub.1 sensed by the up-stream coil 14 of the sensor tube 13 and the, temperature T.sub.2 sensed by the down-stream coil 15 is direct proportional to the current velocity difference of the gas between the up-stream coil 14 and the down-stream coil 15.
The minor temperature difference .delta.T between the up-stream coil 14 and the down-stream coil 15 is converted into a voltage value corresponding to that temperature difference .delta.T. This voltage value is employed in calculation of the current velocity difference of the gas which passes through the sensor tube 13 and, as a result, it is possible to calculate the total gas flowrate per unit time received by the main body 1 of the MFC.
Meanwhile, the sensor device 4 is easily affected by variation of peripheral conditions, such as peripheral temperature and pressure. Particularly, remarkable variation of the peripheral temperature causes the minor temperature difference .delta.T sensed by the sensor device 4 to result in incorrect determination of the total gas flowrate of the main body 1. Hence, such a remarkable variation of the peripheral temperature makes the temperature difference sensed by the sensor device 4 be of no use. If described in detail, since most of the commercialized MFCs, which are calibrated to be used at a temperature of 0.degree. C. and a pressure of 760 Torr., are generally used at different temperatures and different pressures, the practical accuracy of the sensor device 4 of the MFC occasionally reduced to about 5-25% of ideal accuracy and causes incorrect calculation of total gas flowrate of the MFC.
In an effort to overcome the above problem, there has been proposed several techniques, such as installation of overlapped thermal insulation plates about the case 12 and addition of an external case 12 to the sensor device 4. However, these known techniques have a problem in that they only provide limited efficiency, and furthermore, can not provide said limited efficiency when they are employed in a small sized sensor device.